Solidification microstructure of aggregate molded shaped castings

ABSTRACT

A shaped metal casting made in an aggregate mold comprises fine solidification microstructure that is finer than the solidification microstructure of an analogous metal casting made from conventional molding processes. The solidification microstructure may be up to five times finer than the solidification microstructure of a conventionally prepared casting. In preferred embodiments, as a result of directional solidification, the fine solidification microstructure is substantially continuous from a distal end of the casting to a proximal end of the casting, and exhibits greatly enhanced soundness. Because of the control of the uniformity of freezing of the casting, its properties are substantially uniform.

This application is a continuation of application Ser. No. 11/505,019filed on Aug. 16, 2006, which is still pending.

FIELD OF THE INVENTION

The present invention relates to metal castings. More particularly, thepresent invention relates to aggregate shaped metal castings having afine solidification microstructure.

BACKGROUND

In the traditional casting process, molten metal is poured into a moldand solidifies, or freezes, through a loss of heat to the mold. Whenenough heat has been lost from the metal so that it has frozen, theresulting product, i.e., a casting, can support its own weight. Thecasting is then removed from the mold.

Different types of molds of the prior art offer certain advantages. Forexample, green sand molds are composed of an aggregate, sand, that isheld together with a binder such as a mixture of clay and water. Thesemolds may be manufactured rapidly, e.g., in ten (10) seconds for simplemolds in an automated mold making plant. In addition, the sand can berecycled for further use relatively easily.

Other sand molds often use resin based chemical binders that possesshigh dimensional accuracy and high hardness. Such resin-bonded sandmolds take somewhat longer to manufacture than green sand molds becausea curing reaction must take place for the binder to become effective andallow formation of the mold. As in clay-bonded molds, the sand can oftenbe recycled, although with some treatment to remove the resin.

In addition to relatively quick and economical manufacture, sand moldsalso have high productivity. A sand mold can be set aside after themolten metal has been poured to allow it to cool and solidify, allowingother molds to be poured.

The sand that is used as an aggregate in sand molding is most commonlysilica. However, other minerals have been used to avoid the undesirabletransition from alpha quartz to beta quartz at about 570 degrees Celsius(° C.), or 1,058 degrees Fahrenheit (° F.), that include olivine,chromite and zircon. These minerals possess certain disadvantages, asolivine is often variable in its chemistry, leading to problems ofuniform control with chemical binders. Chromite is typically crushed,creating angular grains that lead to a poor surface finish on thecasting and rapid wear of tooling. Zircon is heavy, increasing thedemands on equipment that is used to form and handle a mold and causingrapid tool wear.

In addition, the disadvantages created by the unique aspects of silicaand alternative minerals, sand molds with clay and chemical binderstypically do not allow rapid cooling of the molten metal due to theirrelatively low thermal conductivity. Rapid cooling of the molten metalis often desirable, as it is known in the art that such cooling improvesthe mechanical properties of the casting. In addition, rapid coolingallows the retention of more of the alloying elements in solution,thereby introducing the possibility of eliminating subsequent solutiontreatment, which saves time and expense. The elimination of solutiontreatment prevents the quench that typically follows, removing theproblems of distortion and residual stress in the casting that arecaused by the quench. Related to the mechanical properties, the finenessof the cast microstructure is related to the rate of cooling andsolidification. Generally, as the rate of cooling and solidificationincreases, the solidification microstructure of the casting becomesfiner.

As an alternative to sand molds, molds made of metal or semi-permanentmolds or molds with chills are sometimes used. These metal molds areparticularly advantageous because their relatively high thermalconductivity allows the cast molten metal to cool and solidify quickly,leading to advantageous mechanical properties in the casting. Forexample, a particular casting process known as pressure die castingutilizes metal molds and is known to have a rapid solidification rate.Such a rapid rate of solidification is indicated by the presence of finedendrite arm spacing (DAS) in the casting. As known in the art, thefaster the solidification rate, the smaller the DAS. However, pressuredie casting often allows the formation of defects in a cast part becauseextreme surface turbulence occurs in the molten metal during the fillingof the mold. The presence of fine dendrite arm spacing may also beachieved by cooling the casting by a local chill or fin. Such techniquesinclude the localized application of solid chill materials, such asmetal lump chills or moldable chilling aggregates, and the like, thatare integrated into the mold adjacent to the portion of the casting thatis to be chilled. These methods, however, only provide a localizedeffect in the region where the chill is applied. This localized effectcontrasts with the benefits of the invention discussed in thisapplication, in which the benefits of fine microstructure can apply, ifthe invention is implemented correctly, extensively throughout thecasting. This is an important aspect of the current application, becausethe ultimate benefit is that the casting displays properties that arenot only generally higher, but are also essentially uniform throughoutthe product, and thus of great benefit to the designer of the product.The product now essentially enjoys the uniformity normally associatedwith forgings.

One variety of known permanent mold process in which the residual liquidphase in the structure may be subject to rapid cooling includes sometypes of semi-solid casting. In this process the metallic slurry isformed exterior to the mold, and consists of dendritic fragments insuspension in the residual liquid. The transfer of this mixture into ametal die causes the remaining liquid to freeze quickly, giving a finestructure, but surrounded by relatively coarse and separated dendrites,often in the form of degenerate dendrites, rosettes, or nodules.

However, all molds made from metal possess a significant economicdisadvantage. Because the casting must freeze before it can be removedfrom the mold, multiple metal molds must be used to achieve highproductivity. The need for multiple molds in permanent mold castingincreases the cost of tooling and typically results in costs for toolingthat are at least five (5) times more than those associated with sandmolds.

Another common feature of the internal structure of conventional shapedcastings, well known and well understood throughout the castingindustry, is that those regions of larger geometric modulus (i.e.,regions having a larger ratio of volume to cooling area) generally havea coarser structure. Such regions of the casting typically havesignificantly lower mechanical properties. Further, such regionscommonly exhibit shrinkage cavities or pores because they are moreeasily isolated from feed metal at a late stage of freezing. Suchregions are often seen, for instance, at hot spots formed by an isolatedboss on a relatively thin plate, or in the hot spot that is found at theT-junction between two similar sections. Complicated castings are oftenfull of such features, resisting the attainment of any degree ofuniformity of properties. This problem greatly complicates the work ofthe designer of the product. For instance, the thickening of a sectionintended to increase its strength will lower properties and in the worstinstance may even lead to defects, and so is, often to someindeterminate degree, counter productive.

In the locations of the casting where solidification is slow, not onlyis (i) the structure coarse, typified by a coarse DAS, but (ii) porosityis also present, and (iii) for those Al alloys that commonly suffer ironas an impurity, large plate-like crystals of iron-rich phases can form.All these factors are greatly damaging to the ductility of the alloy.

Extremely fine cast microstructures have been produced as laboratorycuriosities in various scientific studies (for instance, the researchpaper by G. S. Reddy and J. A. Sekhar in Acta Metallurgica, 1989, vol.37, Number 5, pp. 1509-1519 and the research paper by L. Snugovsky, J.F. Major, D. D. Perovic and J. W. Rutter; “Silicon segregation inaluminium casting alloy” Materials Science and Technology 2000 16125-128.).

However, in contrast to such laboratory studies, the invention describedin this application provides the unique conditions in which thesolidification microstructures described herein are produced routinelyby a production process that can be operated to produce one-off orvolume-produced castings that are shaped in three-dimensions.

It is less generally known that the interior of castings can experienceaccelerated freezing as a result of a geometrical effect in shapedcastings. The early solidification near the skin of the casting occurssubstantially unidirectionally, and typically varies at a rate thatdecreases parabolically with time; i.e. the solidification rate reducesin speed as the thickness of the solidified layer increases. Incontrast, the volume of remaining liquid in the center of a castingdwindles with time, and experiences increasing heat extraction fromadditional directions, so that the speed of freezing can be greatlyincreased. This effect is well described by one of the inventors. SeeCastings, John Campbell, pp. 125-126 (2^(nd) Edition, 2003), publishedby Butterworth Heinemann, Oxford, UK, the entire disclosure of which isincorporated herein by reference. The behavior explains the so-calledreverse chill effect in cast iron castings, in which the center of thecasting, seemingly inexplicably, sometimes exhibits a white, apparentlychilled, structure in contrast to the outer regions of the casting thatremain grey, signifying a slow cooling rate.

As a result, it is desirable to develop a casting process and relatedapparatus that have the advantage of rapid solidification of metalmolds, while also having the lower costs, high productively andreclaim-ability associated with sand molds.

It is also desirable to provide an aggregate molded shaped castingexhibiting a region of fine solidification microstructure over extensiveregions of the casting, so as to promote substantially uniformproperties akin to those of forgings. (Because of the relativeinsensitivity of properties to the variations in cooling rate at thehigh cooling rates used in this application, the variations that arediscussed later, for instance in FIG. 4, do not significantly affectproperties, conferring substantial uniformity of properties in the castproduct). In particular, it is desirable to provide an aggregate moldedshaped casting having a fine solidification microstructure that is finerthan a structure produced by a conventional aggregate casting method,and possibly even finer than that produced by a permanent mold.

It is further desirable to provide an aggregate molded shaped castinghaving a fine solidification microstructure region that is substantiallycontinuous from a distal point of the casting to the feeder or riser.

SUMMARY

The disclosure provides, in various embodiments, a shaped metal castingformed in an aggregate mold by an ablation casting process, the castingcomprising a fine solidification microstructure that is finer than themicrostructure of a casting of a similar metal having a similar weightor section thickness that is produced by a conventional aggregatecasting process, wherein the fine microstructure comprises one or moreof grains, dendrites, eutectic phases, or combinations thereof.

The disclosure also provides, in various embodiments, a metal castingexhibiting a cast microstructure, the microstructure comprising a firstregion located adjacent a surface of the metal casting, the first regioncomprising a coarse solidification microstructure; and a second regionlocated internal to the first region, the second region comprising afine solidification microstructure.

In a further aspect, the disclosure provides, in various embodiments, ametal casting formed from a eutectic-containing alloy, the castingcomprising a dual solidification microstructure region, wherein the dualsolidification microstructure region comprises (i) one or more regionscontaining coarse dendrites; and (ii) one or more regions containingfine eutectic.

Additionally, the disclosure provides a shaped metal casting made in amold that is at least a partially aggregate mold, the casting comprisinga dual solidification microstructure region, wherein the dualsolidification microstructure region comprises at least one coarsesolidification microstructure portion having a grain size and/ordendrite arm and/or eutectic spacing in the range commonly to beexpected in a conventional aggregate or metal mold; and at least onefine solidification microstructure having a grain size and/or dendritearm and/or eutectic spacing of less than one third of the conventionalspacing for that portion of the casting.

In still another aspect, the disclosure provides a shaped metal castingmade in an aggregate mold, the casting comprising a dual solidificationmicrostructure region, wherein the dual solidification microstructureregion comprises: at least one coarse solidification microstructureportion having a dendrite arm spacing in the range of about 50 to about200 micrometers; and at least one fine solidification microstructureportion having a dendrite arm spacing of less than about 15 micrometers.

In another aspect, the disclosure provides a shaped metal casting formedin an aggregate mold by an ablation casting process, the castingcomprising a fine solidification microstructure having a dendrite armspacing that is finer than the dendrite arm spacing of a casting havinga similar metal of a similar weight or section thickness that isproduced by a conventional aggregate molded or permanent molded castingprocess.

Further, the disclosure provides a shaped metal casting with substantialsoundness and with high and substantially uniform properties, to someextent resembling those features normally associated with forgings.

Other features and advantages of castings in accordance with thedisclosure are further understood in view of the drawings, detaileddescription, examples, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cooling curve for a solid solution alloy that undergoesdendritic solidification;

FIG. 1A is a graph showing the relationship between dendrite arm spacingand freezing or solidification rate;

FIG. 2 is a micrograph (at ×200) showing the microstructure of a solidsolution alloy cast by conventional casting methods;

FIG. 3 is a micrograph of a solid solution alloy comprising a finesolidification microstructure region (Dual DAS structure) produced byablation;

FIGS. 4A-4E are schematic representations of metal castings comprisingfine solidification microstructure regions;

FIG. 5 is a cooling curve of a mixed dendrite and eutectic alloy;

FIG. 6 is a micrograph (at ×200) depicting the microstructure of a 356alloy showing coarse eutectic silicon particles cast by conventionalmethods;

FIG. 7 is a micrograph (at ×200) depicting an ablation-frozen A356 alloyhaving regions of coarse plus fine dendritic material (dual DASstructure) and fine eutectic material;

FIG. 8 is a micrograph (at ×200) of a portion of an ablation-frozen A356alloy showing uniform coarse DAS and uniform fine eutectic phase;

FIG. 9 is a micrograph (at ×200) of an ablation-frozen A356 alloy havingfine eutectic regions, but which are somewhat coarsened after a solutionheat treatment;

FIG. 10 is a micrograph (at ×200) of an ablation-frozen A356 alloyexhibiting coarse dendritic microstructure and mainly fine eutecticmicrostructure, but containing a trace of coarse eutectic phase;

FIG. 11 is a detail of the micrograph (at ×1000) of FIG. 10 at highermagnification;

FIG. 12 depicts the solidification rate of various portions of adendritic alloy formed in accordance with an exemplary embodiment;

FIG. 13 is a graph depicting the cooling rate of various sections of anexemplary embodiment casting;

FIG. 14 is a table showing the mechanical properties of variousexemplary embodiment castings;

FIG. 15 is a bar graph and table comparing the mechanical properties ofmetal castings formed by various casting methods;

FIG. 16 is a chart showing the relationship between dendrite cell sizeand solidification rate for aluminum alloys;

FIG. 17 a is a micrograph (at ×100) of a conventional cast permanentmold microstructure of a 2.75″ diameter bar of an aluminum alloy;

FIG. 17 b is a micrograph (at ×100) of such a microstructure made withthe ablation process for the same alloy at the start of the ablated part(first part in); and,

FIG. 17 c is a micrograph (at ×100) of the ablated last part.

The figures are merely for the purpose of illustrating variousembodiments of a development in accordance with the present disclosureand are not intended to be limiting embodiments of the development.

DETAILED DESCRIPTION

The disclosure relates to an aggregate molded, shaped casting comprisingat least a fine solidification microstructure region. An aggregatemolded shaped metal casting in accordance with the disclosure includes asolidification microstructure region that is finer than thesolidification microstructure obtained by conventional aggregate moldingmethods. In some embodiments, an aggregate molded shaped metal castingin accordance with the disclosure has a solidification microstructurethat is substantially free of shrinkage porosity.

The type or nature of the solidification microstructure will vary anddepend on the metal and/or metal alloys undergoing solidification.Various microstructures include dendrites, eutectic phases, grains, andthe like. In one embodiment, for example, a shaped casting may compriseonly a single type of microstructure. Further, an alloy may exhibit asolidification microstructure comprising one or more differentmicrostructures. For example, in one embodiment, a shaped casting mayexhibit a microstructure comprising a combination of dendrites andgrains. In another embodiment, a shaped casting may exhibit acombination of dendrites and eutectic phases. In still anotherembodiment, a shaped casting may exhibit a combination of dendrites,eutectic phases, and grains. These embodiments are not limitingembodiments, as other combinations and/or other microstructures may bepossible.

As used herein, eutectic alloys includes any alloy that forms eutecticphases, including hypo-eutectic, near-eutectic or hyper-eutectic alloys.

An aggregate molded, shaped metal casting in accordance with the presentdisclosure may be formed by a method such as that described in U.S.application Ser. No. 10/614,601 that was filed on Jul. 7, 2003, and theentire disclosure of which is incorporated herein by reference.Generally, application Ser. No. 10/614,601 discloses a process for therapid cooling and solidification of aggregate molded shaped castings.The method also provides for the removal of the mold. The processdescribed in application Ser. No. 10/614,601 is referred to herein in as“ablation.”

Upon solidifying during an ablation process, a metal casting exhibits afine solidification microstructure that is finer than the microstructureof a similar metal having a similar weight or section thickness that isproduced by a conventional aggregate casting process. The fineness of amicrostructure may be defined in terms of the size or spacing exhibitedby a particular type of microstructure. For example, grains exhibit agrain size, dendrites exhibit a dendrite arm spacing, and eutecticphases exhibit a eutectic spacing.

With reference to FIG. 1, a cooling or solidification curve for a solidsolution alloy is shown. Solid solution type alloys form only grainsand/or dendrites during solidification. The cooling curve shows thecooling of a solid solution alloy with time, from the pouringtemperature (T_(P)) through the liquidus temperature (T_(L)) to thesolidus temperature (T_(S)), which is the point at which solidificationis complete.

Cooling of a solid solution type alloy in a conventional aggregate moldis represented by the time/temperature profile “abcdef”. The total timefor cooling using conventional methods is in the range of from minutesto hours and is, of course, particularly dependent on the thickness ofthe casting, and the rate at which heat can transfer into the mold. Therate of cooling slows at the liquidus temperature (T_(L)) as a result ofthe latent heat evolution during the formation of dendrites.Solidification is complete at the solidus temperature (T_(S)) and therate of fall of temperature increases once the evolution of latent heathas subsided.

At point “e” on FIG. 1, the application of any rapid cooling is too lateto have any effect on the solidification microstructure. Thus, forexample, the cooling profile “el” would not have any effect on thesolidification microstructure and is not a part of this patentapplication. Cooling profiles such as “el,” however, are commonlyutilized in the casting industry where castings are taken from a metaldie and quenched directly into water.

The solidified structure of solid solution alloys typically consistsalmost entirely of dendrites that are outlined by a negligible thicknessof residual inter-dendritic material. The secondary dendrite arm spacing(often referred to more simply in this application as dendrite armspacing, or DAS) is dependent on the freezing time, i.e. thesolidification time, which is the time that a fixed location in thecasting exists at a temperature between the liquidus T_(L) and thesolidus temperature T_(S) of the alloy. In terms of the period of timein FIG. 1 a, it is seen to be t_(s)=t_(e)−t_(c).

FIG. 1 a shows the approximate logarithmic relation between the localDAS and the local t_(s) in many common Al alloys. This figureillustrates that to reduce DAS by a factor of 10, t_(s) is required tobe reduced by a factor of approximately 1000. (Unfortunately such arelationship has not been investigated for grains and eutectic spacing,so that a clear, quantitative description of the refinement of theseother features of the solidification structure of some alloys cannoteasily be made. Thus the quantitative predictions of refinement ofstructure by ablation described in this application concentrate on DAS.However, it is to be understood that similar but unquantifiedrefinements are paralleled in grain size and eutectic spacing) Thus verylarge increases in cooling rate are required to substantially affect thefineness of the solidification microstructure.

While the quantitative relationship exemplified in FIG. 1 is describedwith respect to dendrites and dendrites arm spacing, similar parallelrefinements are expected in grain size and/or eutectic spacing in alloyscomprising grains and/or eutectic phases.

For conventional aggregate casting methods, for a small aluminum alloycasting weighing a few pounds or kilograms, the local solidificationtime is typically of the order of about 1,000 seconds. Theseconventional aggregate casting methods produce castings having a DAS ofaround 100 micrometers, and sometimes in the range of from about 50micrometers to about 200 micrometers. As used herein, a solidificationmicrostructure having a DAS of greater than 50 micrometers is referredto as “coarse” microstructure. FIG. 2, is a micrograph depicting thecoarse microstructure of a solid solution cast alloy A206 (a nominalAl-4.5 wt % Cu alloy) that was cast by conventional methods.

A casting in accordance with the present disclosure comprises thepresence of fine solidification microstructure in at least a portion ofthe casting. That is, a casting may comprise from greater than 0% to upto 100% of fine solidification microstructure. In one embodiment, thecasting is substantially free of any coarse solidificationmicrostructure and comprises up to 100% fine solidificationmicrostructure that is continuous throughout the casting. In anotherembodiment, a casting comprises a first region adjacent the surface ofthe casting that comprises up to 100% coarse solidificationmicrostructure, and a second region internal to the first region whereinthe second region comprises up to 100% fine solidificationmicrostructure. In still another embodiment, a casting in accordancewith the present disclosure comprises a continuous or at least asubstantially continuous, region of fine solidification microstructureextending from a distal end of the casting to a proximal end thereof,i.e., the end adjacent the feeder or riser.

In other embodiments, a casting comprises a dual solidificationmicrostructure region intermediate a coarse solidificationmicrostructure region and a fine solidification microstructure region.As used herein, a dual microstructure region is a region that includesareas of coarse microstructure having one or more areas of finemicrostructure interspersed therein. In still another embodiment, thedual solidification microstructure in a casting is substantiallycontinuous throughout from a distal end of the casting to the feeder.

The dendrite arm spacing is generally dependent on the time over whichsolidification occurs. As shown in FIG. 1A, the log/log relationship ofdendrite arm spacing to freezing time is linear and, for aluminumalloys, for example, has a slope of approximately ⅓. The graph showsthere is approximately a factor of 5 spacing reduction for each factorof 100 in freezing time. Thus, a particular casting section in aconventional mold may experience solidification in 1000 seconds giving acorresponding DAS of 100 micrometers. In the ablation casting techniquesdescribed in U.S. application Ser. No. 10/614,601, the same castingsection would result in a local solidification time of onlyapproximately 10 seconds, giving a dendrite arm spacing of about 20micrometers. The relationship between spacing and freezing time remainsconstant over all experimental times. FIG. 3 is a micrograph showingregions of both fine microstructure and regions of coarsemicrostructure.

With reference to FIGS. 4A-4E, various exemplary embodiments ofaggregate molded, shaped metal castings comprising a fine solidificationmicrostructure region are shown.

With reference to FIG. 4A, an embodiment is shown in which a castingcomprises a small percentage of fine solidification microstructure. Thecasting in FIG. 4A represents a situation in which the rapid coolingprocess, such as ablation, is applied late in the casting process. Inthe casting in FIG. 4A, some solidification has occurred prior to rapidcooling (e.g., ablation), and portions of the casting, such as thosehaving a small geometric modulus (volume to cooling area ratio)conventionally freeze. Dual solidification microstructure regions occurwhere some portions have solidified prior to ablation but other portionsremain liquid at the time ablation begins Even in an example such asFIG. 4A where a portion of the casting has solidified prior to a rapidcooling process, and thus constituting a far from optimum application ofthis invention, the presence of even a small amount of finesolidification microstructure is advantageous. Specifically, inconventional processes, the small percentage of residual liquid in metalalloys that remains is particularly difficult to freeze without creatingdefects such as shrinkage porosity. Applying a rapid cooling techniquesuch as ablation, however, converts these regions from defective zonesto fine structure zones. The presence of even small zones of finestructure provide good mechanical properties compared to those defectivezones of a conventionally solidified casting. Although the trappedregion of residual liquid illustrated in FIG. 4A will be expected todemonstrate some shrinkage porosity, ablation freezing of this regionwill reduce the extent of the shrinkage, and will replace it with acorresponding region of strong, sound material. The solidificationmicrostructure profile of FIG. 4A would be expected where the rapidcooling step is applied rather late, closer to, but before, point “e” onthe graph of FIG. 1.

In another embodiment, an aggregate molded shaped casting comprises afine solidification microstructure region and a dual solidificationmicrostructure region wherein the dual solidification microstructureregion is substantially continuous from a distal end of the casting to aproximal end of the casting. The embodiment of FIG. 4B is an example ofan embodiment of a casting having a substantially continuous dualsolidification microstructure region. The solidification microstructureprofile of FIG. 4B is a profile that would be expected from followingthe cooling profile “abcdjk” in FIG. 1. In FIG. 4B, the freezing pointarrives at and passes the point at which the natural freezing of theconstricted section reaches the center of the section. The casting isfrozen by a rapid cooling procedure, such as ablation, from the moredistant parts of the casting up to the center point. If the finestructure zone produced by rapid freezing is terminated on reaching themodulus constriction (as it does in the embodiment in FIG. 4B) thecasting will freeze soundly up to this point. Even though the dualsolidification microstructure is absent in the local region of theconstriction in the embodiment in FIG. 4B, the rapid localsolidification time throughout the remainder of the casting creates asubstantially continuous zone of fine and sound alloy, free fromshrinkage defects, throughout the remainder of the casting. Thus, bydriving the solidification directionally from distal regions to thoseproximal to the feeder, the more distant portions of the casting exhibitfine solidification microstructure and mechanical soundness that wouldnot have been achieved by conventional methods.

In the embodiment in FIG. 4C, the casting includes a greater percentageof fine solidification microstructure, and the region of dualsolidification microstructure is continuous through the constrictedregion of the casting. Such a desirable solidification microstructuremay be achieved by applying a rapid cooling procedure, such as ablation,at a time earlier than that of FIG. 4B.

In another embodiment, such as that of FIG. 4D, a metal castingcomprises a desirable fine solidification microstructure region that issubstantially continuous from the distal ends of the casting to thefeeder. Such a solidification microstructure may be achieved by applyinga rapid cooling procedure early in the cooling profile. In thisembodiment, the casting may also include regions of dual solidificationmicrostructure and coarse solidification microstructure. Thesolidification microstructure profile of FIGS. 4C and 4D would beexpected from applying a rapid cooling at an early time, such that thenarrowest part of the casting would follow a path starting at a pointbetween c and d on the cooling profile in FIG. 1.

In still another embodiment, such as the embodiment of FIG. 4E, theentire solidification microstructure comprises a fine solidificationmicrostructure. Such a desirable structure might be achieved by applyinga rapid cooling method at Point “b” in the cooling curve of FIG. 1 andfollowing the profile “abghi.” This would occur if no freezing occursdue to loss of heat to the mold and the freezing occurs totallyunidirectionally and at a high rate. Such a structure, however, is noteasily achieved and has yet to be experimentally achieved by theinventors. Difficulties in achieving this solidification microstructurearise from the impingement of the liquid coolant directly on the surfaceof the still liquid casting. A casting comprising 100% finesolidification microstructure may be achievable under certain conditionssuch as using a highly insulating mold, and applying a highlydirectional solidification process.

An aggregate molded, shaped metal casting may comprise from about 1 toabout 100% fine solidification microstructure. Even a small amount ofsolidification microstructure is desirable for enhancing the mechanicalproperties of a casting. This is especially the case where the creationof small amounts of fine solidification microstructure, denoting as itdoes in this invention the action of directional solidification, andthus optimal feeding, prevents defects such as shrinkage porosity fromoccurring in the casting.

Castings in accordance with the present disclosure that include a finesolidification microstructure region may be formed from any solidsolution alloy that solidifies dendritically. These include both ferrousmaterials and non-ferrous materials. The dendrite arm spacing of boththe coarse and fine solidification microstructure regions will varydepending on the metal that is used. With respect to aluminum alloys,coarse solidification microstructure regions typically have a dendritearm spacing of greater than about 50 micrometers. In some embodimentsthe coarse solidification microstructure has a dendrite arm spacing offrom about 50 to about 200 micrometers. Also in aluminum alloys, thefine solidification microstructure has a dendrite arm spacing of lessthan about 15 micrometers, and, in some embodiments, is from about 5 toabout 15 micrometers.

Alloys in which solidification occurs partly by dendritic solidificationand partly by eutectic solidification, as is typical of many Al—Sialloys, exemplified by Al-7Si-0.4Mg (A356) alloy, may also exhibit finedendritic and/or fine eutectic microstructure. The conventional coolingcurve for mixed dendrite/eutectic alloys is illustrated in FIG. 5 ascurve “a-h.” Starting at the pouring temperature (T_(P)) at point “a”the liquid alloy cools to the liquidus temperature (T_(L)), at point“c,” which is the point at which dendrites start to form. The dendritegrowth is complete at point “e,” which is the eutectic temperature(T_(E)). The fall in temperature is arrested, forming a plateau untilthe completion of the eutectic solidification at point “g”. At thispoint the casting is completely frozen, and further cooling to roomtemperature follows “gh.” A second example alloy of an Al—Si alloy thatbenefits powerfully from the application of this invention is the widelyused A319 alloy. This alloy also contains some copper. The alloy differssomewhat from A356 in having a eutectic formation region “eg” that isnot isothermal, the horizontal plateau “eg” of FIG. 5 being replaced bya steady downward slope. However, the same principles apply precisely.

This slow conventional cooling in, for instance, a silica sand mold,results in a structure of dendrites, having a DAS in the region of 200down to 50 micrometers. The dendrites are surrounded by eutectic, whichis characterized by a spacing in the region of 20 down to 2 micrometers.This is denoted the conventional or “coarse” eutectic microstructure forpurposes of our description (FIG. 6).

If it were possible to subject the alloy to total cooling by ablation,the cooling profile would follow the path “bijkl”′ so that the wholesolidification microstructure would consist of fine dendrites and veryfine eutectic. However, as discussed above, although this structure isnot easily obtained, and has yet to be tested by the inventors, it maybe achievable in special conditions. These conditions might includecircumstances in which the mold is highly insulating, and the freezingis highly directional. Castings having excellent mechanical propertiesare, nevertheless, achievable without resort to these specialconditions.

In general, a more practical cooling profile is illustrated by thecooling curve “abcdmno.” In this situation the prior cooling from “cd”creates coarse dendrites to strengthen the casting in its hot, partiallysolidified, and therefore weak state, prior to the application of thecoolant. The subsequent dendrites and the eutectic are both thensubjected to rapid cooling, so that the fine solidificationmicrostructure includes both fine dendrites (DAS in the region of 30down to 5 micrometers) and fine eutectic (not resolvable at 1000×magnification, since its spacing is around only 1 micrometer).

The two regions of the consequently dual microstructure form visuallyhighly distinct areas when viewed under the microscope. FIG. 7 shows astructure in which the ablation was applied in time to freeze somedendritic material, followed by the rapid freezing of all of theeutectic. The eutectic is so fine that it is not resolvable in thisimage, but appears as a uniform light grey phase (in this case the alloyhad no refining action of the additions of chemical modifiers such as Naor Sr). Additionally, because all of the eutectic freezes along the path“mn,” the whole of the eutectic phase, between both the coarse and thefine dendrites, is seen to be uniformly fine in FIG. 7.

The uniform and extremely fine eutectic is a common feature of ablatedsolidification microstructures and is unique to ablation cooled alloysthat have received no benefit from chemical modification by Na or Sr asan aid to refine the microstructure. It is seen in FIG. 8, in which somefinely distributed dross and associated pores can also be seen in thestructure. The mechanical properties of the castings appear to beremarkable insensitive to most defects of this variety and size. FIG. 9illustrates a similar fine eutectic after a solution heat treatment. InFIG. 9, the eutectic has coarsened somewhat to reduce its interfacialenergy as is common for two-phase structures submitted to hightemperature treatment.

In principle, although not normally desirable, it would be possible toallow all of the dendrites to freeze with the coarse structure, makingonly a late application of the ablation cooling, such as, for instance,at point “f” in FIG. 5. In this case some of the eutectic would havefrozen with a coarse eutectic structure. Such a structure is shown inFIG. 10. The final regions of the eutectic that freezes with the benefitof ablation cooling adopt the extremely fine structure and are generallyfree from porosity and exhibit only fine iron-rich phases, which aregenerally too small to be seen in FIG. 10. At higher magnification, afew iron-rich phases can be seen, as shown in FIG. 11.

For mixed dendrite/eutectic alloys, most of the benefits of ablation areenjoyed by those structures seen in FIGS. 10 and 11 because progressivesolidification of the residual liquid will still be effective to feedthe casting directionally. The alloy in FIG. 10, for example, has beenheat treated, therefore to some extent coarsening all of the siliconparticles in the eutectic phase.

In practice, however, it is desirable, and easily achieved, for somecoarse dendritic structure to be formed prior to the application of theablation (starting point “d” in FIG. 5). The usual resulting dendriticmicrostructure is therefore dual in the sense described above andincludes relatively uniformly fine eutectic.

As before, if coolant is applied extremely late, for instance followingpath “gq” in FIG. 5, the action of ablation cannot influence thesolidification microstructure of the casting because, of course, thecasting has fully solidified prior to any application of a coolant. Suchcooling of a casting does not form part of this patent application, andfalls into the casting production regime well known to those skilled inthe art.

For conventionally cooled castings (those that adopt the cooling pathending in “h” or “q”) the last regions of the casting to solidify oftencontain porosity. In addition, such castings when made in a typicalaluminum alloy such as A356 alloy, often contain thin platelets ofbeta-iron precipitates that further impair properties.

Ablation-cooled castings, including both dendritic castings anddendritic/eutectic castings, comprising a fine solidificationmicrostructure are generally free of defects that are often found incastings formed by conventional casting methods. In one embodiment, acasting comprising a fine solidification microstructure portion issubstantially free from porosity. The rapid freezing and directionalfeeding created by ablation reduces both gas and shrinkage porosity. Inanother embodiment, a casting comprising a fine solidificationmicrostructure portion is substantially free of large damaging iron-richplatelets. In other embodiments, a casting is substantially free of bothporosity and iron-rich platelets. Without being bound to any particulartheory, the reduction in size of the iron-rich platelets may be theresult of the more rapid quench of the liquid alloy. The reduction ofporosity also benefits from this speed. In addition, it is significantlyaided by the naturally progressive action of the ablation process, inwhich the cooling action of water (or other fluid) is moved steadilyalong the length of the casting to drive the solidification in a highlydirectional mode towards the source of feed metal. Furthermore, themaintenance of a relatively narrow pasty zone by the imposition of ahigh temperature gradient in this way is highly effective in assistingthe feeding of the casting.

The substantial reduction or elimination of shrinkage porosity issignificant, and may be restated as follows. Shrinkage porosity wouldnormally be expected in regions of the casting such as an unfed hotspot. In principle, however, these regions can be fed if the freezingprocess is carried out directionally. The water or other cooling fluidis applied to ablate the mold and cool and cause solidification in thecasting progress systematically, creating a uniquely strong directionaltemperature gradient. Thus, those regions that would have been isolatedfrom feed liquid in a conventional casting are easily and automaticallyfed to soundness, or greatly improved soundness, when the benefits ofthe invention are correctly applied.

For this reason, alloys that cannot normally be cast as shaped castingsbecause of hot-shortness problems, such as the wrought alloys 6061 and7075, etc., or with long freezing range such as alloys 7075 and 852, caneasily and beneficially be cast into a shaped form via ablationtechniques. In addition, the ablated castings are characterized by asolidification microstructure that is immediately identifiable as beingunique in a shaped casting.

An aggregate molded shaped metal casting comprising a finesolidification microstructure region is further described with referenceto the following examples. The examples are merely for the purpose ofillustrating potential embodiments of a shaped metal casting having afine solidification microstructure region and are not intended to belimiting embodiments thereof.

EXAMPLES

In one example of the application of the technique described in thisapplication, a single test bar was molded of diameter 20 mm and length200 mm furnished with a small conical pouring basin at one end that wasfilled to act as a feeder. The mold material was silica sand bonded witha water-soluble inorganic binder as described in our co-pendingapplication Ser. No. 10/614,601.

Thermocouples were inserted into the mold cavity at the base of thefeeder, and at the base of the cavity. Two additional thermocouples werelocated at equal intervals along the axis. These four thermocouples werelabeled TC1 (riser), TC2 (top midsection), TC3 (bottom midsection) andTC4 (bottom).

An aluminum alloy 6061 at a temperature of 730° C. (1350° F.) was pouredinto the cavity, arranged with its axis vertical. Within approximately10 seconds, water at 20° C. (68° F.) was then applied from nozzlesdirected at the base of the mold so as to start the ablation of the moldfrom the base upward. The rate of upward progression of the ablationfront was approximately 25 mm/s

Cooling traces of the four thermocouples were recorded as seen in FIG.12. The thermocouple TC4 is seen to cool rapidly, signaling the freezingand cooling to below the boiling point of water in only about 2 seconds.At this time the thermocouple immediately above, TC3, still records thatthe metal is still molten, and that cooling has only just begun. Thispattern is repeated successively up the mold. (The jump in temperaturefor TC2 records the unintentional momentary loss of cooling water inthis experiment). The thermal traces confirm that the temperaturegradient caused by the application of ablation was sufficient to freezethe melt and cool it to near ambient temperatures within a distance ofless than the spacing between thermocouples (50 mm). Furthermore, theeffect was easily and accurately sustainable for the length of anaverage automotive casting.

In a second example an automotive knuckle casting was made in alloyA356. Many knuckle castings have a reputation for being difficult tocast because of their complexity, having heavy sections distant frompoints where feeders can be added. This casting was no exception. Thecasting was filled with metal at 750° C. (1385° F.) on a tilt pouringstation, taking 8 seconds to fill, resulting in an excellent surfacefinish. The feeder (riser) was located at the far end of the castingfrom the pouring cup and down-sprue. Thermocouples in the sprue andfeeder are illustrated in FIG. 13. It is seen that freezing took placein the sprue, being the first to ablate, in approximately 20 seconds.Freezing was then caused to progress across the casting, arriving at thefeeder approximately 90 seconds later, at which point the feeder iscaused to freeze at a similar time of only about 20 seconds.

To achieve ablation for this casting, three banks of water spray nozzleswere employed, with water pressure at approximately 0.7 bar(approximately 10 psi) and temperature approximately 40° C.(approximately 100° F.).

The casting was found to be completely sound, and with propertiesexceeding specification.

In a third example a control arm casting, a steering/suspensioncomponent of an automobile, was molded in the mold material as specifiedfor Example 1. The mold was poured with A356 alloy of approximatelycomposition Al-7Si-0.35Mg-0.2Fe at approximately 700° C. (approximately1400° F.). Ablation cooling of this mold produced a casting that wassubsequently cut up and machined to produce tensile test bars that weresubject to a T6 heat treatment. The solution treatment was 538° C.(1000° F.) for 0.5 hours, water quench at 26° C. (78° F.) and age at182° C. (360° F.) for 2.5 hours. Four test bars were cut from each ofthree castings numbered 45, 46 and 47. The bars were subjected totensile testing and the results are listed, together with averages, inFIG. 14. (The one low extension value of 9% was attributed to a largeoxide inclusion since the control of the melt quality was known to beless than optimum on this occasion.) The results are compared with thosefrom competitive casting processes in FIG. 15. The properties areclearly attractive, exceeding those of all current best competitiveprocesses.

It should be appreciated that certain potential process conditions couldresult in a conventional microstructure from an ablated mold when usingthe ablation casting process. As an example, an 852 aluminum alloy(Al-6Sn-2Cu-1Ni-0.75Mg alloy) is known as a long range freezing alloy,wherein the eutectic freezes approximately at the melting point of tin(232 C, 610 F). This alloy was ablated using the ablation castingprocess. The mold was symmetrical, allowing the identical mold halves tobe produced from a single sided pattern and then assembled. The metalwas poured at or near 700 C (1275 F). The casting section thickness wasapproximately 75 mm (3 inch) in diameter. The pouring of the mold bygravity was achieved in 10 seconds. The mold was then left to sit for aperiod of nearly 180 seconds, to achieve a mostly solidified alphaphase. After this period of normal solidification rate being controlledby the molding aggregate (in this case silica sand), the mold wasablated.

The ablation conditions were as follows. Water pressure used forablation was approximately 1 bar (15 psi). The spray volume was limitedby the spray nozzles. However, the water volume is nearly insignificantas the pressure controls the water impingement against the as castsurface. This procedure captured a conventional but uniform cooling ratethat yielded similar properties to that produced by a metal mold (i.e.,a permanent mold, see FIG. 16). In this connection, FIG. 16 shows thespectrum of various casting processes and the relationship betweendendrite cell size and solidification rate for aluminum alloys. Aconventional permanent mold microstructure is illustrated in FIG. 17 a.FIGS. 17 b and c show the same alloy but now created using the ablationprocess. In all three of FIGS. 17 a-c, the same magnification, 100×, wasemployed. The final eutectic structure was closely similar to thatproduced by a permanent mold. Although such a conventionalmicrostructure can be achieved in the ablation process, in someconditions, those microstructures unique to ablation, includingextremely fine phases possibly of dendrites, but more often of extremelyfine eutectic, can also be observed.

To use the ablation process to capture a conventional microstructure(such as that resulting from a permanent mold), several variables havesignificant potential. An important parameter is the mold aggregateitself In addition, the volume, pressure and temperature of the coolingmedium used in removing the mold while simultaneously causingsolidification of the metal are, of course, also important. During theiradvance along the length of the casting, the dwell time of the coolingsprays upon the casting can be beneficially adjusted to allow for thelocal surface to volume ratio (the geometrical modulus). The rate ofdissolution of the mold binder can be reduced to slow the rate ofablation of the mold, and so reduce the rate of thermal extraction. Thiscould limit the rate so as to produce a conventional microstructure.Furthermore, the water pressure can be varied. At first, a higherpressure can be used to remove the aggregate of the mold. Then, thepressure can be reduced to create a cooling rate that would be akin tothat of a conventional metal mold process.

Although a conventional microstructure may be attained in this way,there are significant additional benefits offered by the ablationprocess that make the ablation process uniquely desirable. First, thereis improved control of the residual stress within the casting since thefinal solidification occurs with sufficient liquid ahead of the finalsolidification front to allow for a complete accommodation of thermalstrains. Second, the porosity in the casting is significantly reduced(practically to zero in the majority of cases) because of the excellentdirectional solidification. This occurs as a result of the high appliedtemperature gradient that ablation uniquely achieves.

Aggregate molded or shaped castings having time solidificationmicrostructure have been described with reference to the presentdisclosure and various exemplary embodiments. It will be appreciatedthat variations or modifications may be within the capabilities of aperson skilled in the art and that the present application and claimsare intended to encompass such modifications. It is intended that thedevelopment be construed as including all such modifications andalterations insofar as they come within the scope of the appended claimsand the equivalents thereof.

1. A shaped metal casting formed in a mold comprising an aggregate by anablation casting process, the casting comprising a fine solidificationmicrostructure that is finer than the microstructure of a casting havinga similar metal of a similar weight or section thickness that isproduced by a conventional aggregate molded casting process, wherein thefine microstructure comprises one or more of grains, dendrites, eutecticphases or combinations thereof, wherein the fine solidificationmicrostructure is about five times finer than the microstructure of acasting of a similar metal that is produced by a conventional aggregatecasting process.
 2. The shaped metal casting according to claim 1,wherein the casting is substantially free of porosity.
 3. The shapedmetal casting according to claim 1, further comprising a dualsolidification microstructure region comprising (i) a coarsesolidification microstructure portion, and (ii) a fine solidificationmicrostructure portion interspersed within the coarse solidificationmicrostructure portion.
 4. The shaped metal casting according to claim3, wherein the dual solidification microstructure region issubstantially continuous from a distal end of the casting to a proximalend thereof.
 5. A metal casting formed in a mold comprising an aggregatevia a rapid cooling process and exhibiting a cast microstructure, themicrostructure comprising: a first region located adjacent a surface ofthe metal casting, the first region comprising a coarse solidificationmicrostructure; and a second region located internal to the firstregion, the second region comprising a fine solidificationmicrostructure, wherein the second region comprises dendrites having adendrite arm spacing of about 5 to about 15 micrometers, wherein thefirst region comprises dendrites having a dendrite arm spacing of about50 to about 200 micrometers.
 6. The metal casting according to claim 5,wherein the fine solidification microstructure of the second region isfiner than a fine solidification microstructure of a casting of asimilar metal having a similar weight or section thickness that isformed by a conventional aggregate casting process.
 7. The metal castingof claim 5, wherein the casting is substantially free of at least one of(i) shrinkage porosity, and (ii) damaging iron-rich platelets.
 8. Themetal casting of claim 5, wherein the first region comprisesapproximately 100% course solidification microstructure, and the secondregion comprises approximately 100% fine solidification microstructure.9. The metal casting of claim 8 further comprising a third regionlocated between the first and second regions, wherein the third regioncomprises a dual solidification microstructure comprising (i) one ormore coarse solidification microstructure portions, and (ii) one or morefine solidification microstructure portions.
 10. The metal casting ofclaim 9, wherein the one or more coarse solidification microstructureportions of the dual solidification microstructure region comprisesdendrites having a dendrite arm spacing of about 50 to about 200micrometers and the one or more fine solidification microstructureportions of the dual solidification microstructure region comprisesdendrites having a dendrite arm spacing of about 15 micrometers or less.11. A metal casting formed in a mold comprising an aggregate via a rapidcooling process and exhibiting a cast microstructure, the microstructurecomprising: a first region located adjacent a surface of the metalcasting, the first region comprising a coarse solidificationmicrostructure having dendrites with a dendrite arm spacing of 50 to 200micrometers; and a second region located internal to the first region,the second region comprising a fine solidification microstructure havingdendrites with an arm spacing of 30 micrometers or less.
 12. The metalcasting of claim 11, wherein the second region comprises dendriteshaving a dendrite arm spacing of 20 micrometers or less.
 13. The metalcasting of claim 12, wherein the second region comprises dendriteshaving a dendrite arm spacing of 5 to 15 micrometers.
 14. The metalcasting of claim 11, wherein the casting is free of at least one of (i)shrinkage porosity, and (ii) damaging iron-rich platelets.
 15. The metalcasting of claim 11, wherein the first region comprises 100% coarsesolidification microstructure, and the second region comprises 100% finesolidification microstructure.